The proposed studies address the issues of the biochemical nature and physiologic regulation of biologically active forms of prolactin in lactating and extrogen treated rats. Previous reports suggest that there are at least three "pools" of prolactin in the pituitary. A newly synthesized pool of prolactin can be released, presumably most rapidly of any form in the pituitary, or converted to a storage pool which is not directly released from the lactotroph but instead is transformed to a second storage pool from which releases can occur. This transformation process is marked in the lactating rat but unresolved during other physiological conditions. The most interesting feature of the second storage form is that it is often undetectable by many extraction methods. specific experiments are designed (1) to localize these pools with different solubiliztion and centrifugation procedures (2) to describe the controls over movement between these pools during synthesis and release using several different animal models including pregnant, lactating and steroid treated rats (3) to determine if various molecular sized of prolactin relate to these pools as determined by Sephadex gel filtration and polyacrylamide gel electrophoretic techniques (4) to determine what forms of pituitary prolactin are biologically active using the recently developed lymphoma cell bioassay.